Rotary switching device and contact elements therefor



1958 A. J. MONACK ET AL 2,821,584

ROTARY SWITCHING DEVICE AND CONTACT ELEMENTS THEREFOR Filed April 8, 1954 5 Sheets-Sheet 1 Jan. 28, 1958 MQ'NACK ET L 2,821,584

ROTARY SWITCHING DEVICE AND CONTACT ELEMENTS THEREFOR 3 Sheets-Sheet 2 Filed April 8, 1954 P y, W 4 7L & 7 E m/ Jan; 28, 1958 J, MONACK ET AL 2,821,584

ROTARY SWITCHING DEVICE AND CONTACT ELEMENTS THEREFOR Filed April 8 1954 3 Sheets-Sheet 3 United States Patent ROTARY SWITCHING DEVICE AlID CONTACT ELEMENTS THEREFOR Albert J. Monack, Rutherford, N. .L, and Martin V. Kiebert, Jr., Indianapolis, Ind., assignors to Mycalex Electronics Corporation, Clifton, N. J., a corporation of New York Application April 8, 1954, Serial No. 421,882

11 Claims. (Cl. 200-28) This invention relates to switching devices for such purposes as elevator controls, process controls, and the like, and to telemetering commutators, and more particularly to commutators having in combination a wiper arm adapted to contact a brush element with a plurality of electrical contacts. Such commutators are built into guided missiles, aircraft, coding devices, and similar apparatus, or are incorporated into the controlling mechanism thereof.

In such switching or controlling apparatus, as for example guided missiles and related uses, various radio messages or other electrical signals may be received, and may be translated into appropriate forms of action by means of a commutator comprising one or more brush elements disposed on one or more wiper arms and adapted to be moved over a continuous contact and simultaneously over a plurality of interrupted contacts to complete a circuit sequentially between the continuous contact and each of the discontinuous contacts, or selected groups thereof. Since the brushes on the wiper arms are normally in continuous motion over the contact members, and since usually only minute currents are employed, it is very important to eliminate all causes of spurious currents which interfere with the signal transmitted. Such spurious currents can arise, for example, if there is any bouncing of the brushes in their movement over the contacts, or vibration of the wiper arm of sufiicient magnitude to affect the pressure of the brushes and hence their degree of contact resistance.

The brush-carrying element of such a commutator may' type, there is a tendency for the brush-carrier moving successively thereover to vibrate harmonicaily with the rate of movement. Such vibration is detrimental to the operation of the device, since the changing pressure of the brushes or a break in contact produces small transient currents generating noise, and causing variations in the voltage and phase relations of the signal.

These limitations are overcome by the present invention, wherein vibration of the brush-carrier is damped by its novel construction. This damping may be achieved by constructing the carrier of a plurality of leaf springs of different vibratory periods, obtained by using leaves of a given material of (littering dimensions, or leaves of various materials having diiierent spring constants; damping may also be achieved by attaching a damping member to a resilient brush-carrier, or by a combination of any of the foregoing methods.

Another ettective means of lessening vibration is pro vided by this invention, in the use of a switch-plate which firmly holds the contacts in precision spacing, and with their brush-contacting surfaces in an unvarying plane.

In the present invention it has also been found that certain electrical alloys are especially satisfactory for brush materials and that various kinds of plating on the contact members are advantageous; and that some platings are more desirable when brushes of a particular alloy are run against them than with others. Examples of such combinations are given in the following specification.

It is therefore a primary object of the present invention to provide a telemetering commutator in which brushcontact pressure is maintained constant.

It is another object of this invention to provide a commutator wiper arm having a resilient but substantially non-vibratory structure.

A further object of the present invention is the provision of a wiper arm having a plurality of brush-carrying spring elements of difierent vibratory periods.

Yet another object is to provide a commutator hav ing a wiper arm in combination with vibration-clampingmeans.

A still further object of this invention is the provision of a commutator brush-carrier having a resilient element disposed between a plurality of structural elements.

It is another object to provide a switching device having advantageous combinations of contact metals and brush metals.

Yet a further object is the provision of a switclrpiate adapted to hold the contacts tightly in precision spacing and with their brush-contacting surfaces in an unvarying plane.

The foregoing objects and others ancillary thereto will be better understood from the following description of the invention when read in connection with the accompanying drawings, wherein like numbers refer to like parts, and in which:

Fig. 1 is a sectional utility View of the commutator;

Fig. 2 is a plan view of the switch-plate taken on line 2-.2 of Fig. 1;

Fig. 3 is an enlarged view of the wiper arm taken on line 3-3 of Fig. 1;

Fig. 4 is an elevational view of an embodiment of the wiper arm;

Fig. 5 is an elevation of an embodiment of one of the brush-carriers;

Fig. 6 is a bottom View of one of the brush-carriers;

Fig. 7 is an elevation of another embodiment of one of the brush-carriers;

Fig. 8 is an elevation of a third embodiment of one i of the brush-carriers;

connected to a gear-train (not shown) disposed within housing 12. The gear train is in driving relationship to rotatable shaft 13 on which Wiper arm 14 (described more precisely below) is transversely mounted in such a manner that it rotates in a plane normal to the axis of the shaft. Disposed Within the barrel of the housing and seated against an internal step thereof is switch-plate 16, adjacent to the Wiper arm and in a plane parallel to the plane of rotation of said arm. The switch-plate is held in position by retainer ring 17 threadably engaged with the housing; a cushioning gasket 18 is disposed between the retainer ring and the switch-plate. A hearing 19 is embedded in the switch-plate concentric therewith, and adapted to receive the non-driven journal end of shaft 13.

Referring now to Fig. 2, there is shown the switchplate 16, comprising a base plate 21 of insulating material Patented Jan. 28, 1958 having embedded therein the bearing 19, an inner electrically conducting contact ring or slip ring 22, an outer electrically conducting contact ring 23, a first circle of spaced-apart electrical contacts 24 surrounding said outer contact ring, and a second circle of spaced-apart electrical contacts 26 surrounding said first circle, all such contact rings and circles of contacts being concentric with each other and with the base plate 21. An indexing notch 27 of any convenient form may be provided in the edge of the base plate for positioning it within the barrel. Contacts 24 and 26 pass entirely through. the base plate and protrude from the back thereof (as shown in Fig. 1) to permit making electrical connections, and may, if desired, be provided with reduced or expanded diameters, or knurling, to serve as locking means in the insulating material of plate 21. Ring 22 is in electrical contact with pin 28 extending through the base plate, and ring 23 is in electrical contact with a similar pin 29. It will be understood that although the present application discusses only a commutator having provisions for two circuits, the in vention includes commutators having less than two circuits or more than two circuits, and such other embodiments have been made and tested. It will also be apparent that in addition to varying the number of circuits by varying the number of contact rings and associated spaced-apart contacts, it is also possible to vary the position of these elements in any desired manner. A further variation for special purposes is the use of interrupted contacts in place of one or more of the continuous sliprings shown.

A particularly satisfactory insulating material for the composition of switch-plate 16' has been found to be glass-bonded mica. This moldable inorganic material comprises mica particles in suspension in a glass binder, providing an insulator of highly satisfactory electrical properties, and having in addition the desirable mechanical properties of dimensional stability, substantially zero moisture absorption, high resistance to heat and corrosion, and being moldable to precision dimensions. These properties allow the interrupted contacts, slip-rings, and bearing sleeve to be molded in at a single operation, and these elements are then retained in the required precise spacing from each other, and in such a manner that none of the contact faces will deviate from the operating plane under any operating conditions which the rest of the equipment will stand. This characteristic is particularly important in combination with the wiper arm and brushcarriers later to be described, where it is desired to eliminate as far as possible all bouncing of brushes on the contacts, which would result in the generation of spurious signals. The maintenance of precision spacing of the contacts is also highly significant in order that duty cycle and phase relations shall be constant. It is not intended, however, to limit the invention to the use of a switchplate of glass-bonded mica, since other insulating materials may also be used.

Turning now to Figs. 3 and 4, there are shown two views of the wiper arm 14 hearing brush-carriers. A bar 31, of insulating material, metal, or other suitable material is provided with a bore 32 transversely therethrough, of proper diameter to give a close fit with shaft 13. A slot 33 through the bar extends along the longitudinal centerline thereof a sufficient distance on each side of the bore to allow a slight deformation of the bar at the center. Screws 34 pass through the bar across the slot and when tightened provide a means of clamping the bore securely around the shaft. This arrangement has been found satisfactory in practice, but any other means of attaching the bar to the shaft, such as a set screw, soldering, etc. would be equally applicable.

One side of bar 31 is recessed at each end thereof to lay in two plates of insulating material in each recess flush with the surface of the bar. Insulator plate 36 laid in at one end of bar 31 is angularly channeled or grooved to receive the fixed ends of the brush-carrying elements (later to be described). Insulator plate 37 of the same external configuration as plate 36, but without the channel, is then laid on and securely fastened to the plate 36 and bar 31, as by screws 38 or other suitable means, by which the brush-carrier assembly is clamped in place.

In the recess at the opposite end of bar 31 is positioned insulator plate 39, having a similar brush-carrier channel grooved therein at a different angle from that of plate 36. Another brush-carrier assembly is disposed in the said channel. and insulator plate 41 secured on top thereof by screws 42 or other convenient fastening means.

It has been found convenient in manufacture to make identical brush-carrying elements for either end of the wiper arm. It is an important element of the invention that the center-to-center distance between brushes on each brush-carrier assembly be substantially equal, in order that the same damping arrangement may be used on each carrier and thus the dynamics of all carriers will be the same in operation. Where the commutator is used for low voltages and low currents, there are also electrical reasons for maintainingthe same distance between brushes. One brush-carrier assembly bears contacts for the inner slip ring and the first circle of interrupted contacts; the other carrier bears brushes for the outer slip ring and the second circle of contacts. Therefore, it is essential that the angles chosen for channeling plates 36 and 39 be such as to permit equally spaced brushes to reach and contact the selected circuit. Alteration of the phase relations can be obtained by making the angular channel slightly wider than the brush-carrier assembly intended to fit in it, and slightly changing the angle of the assembly in the channel after loosening screws 38 or 42. If desired, the brushes may be oriented variously on the brush-carriers, to present a selected edge or dimension as the leading edge in the travel circle; for present purposes, it has been found satis factory to orient the brushes in such a manner that their longitudinal dimension is tangent to the travel circle.

Referring now to Fig. 5, there is shown one form of the brush-carrier assembly. A first flat elongated plate 43 of springy, electrically conducting material is given a double bend toward one end of its length, to offset it in such a manner that the two ends occupy substantially parallel, spaced-apart planes. A second elongated plate 44 of similar springy, electrically conducting material is positioned in parallel and electrical contact at one of its ends with the shorter end of plate 43, the other end of plate 44 being bent or deflected toward the first plate, so that the bent end of the second plate is in contact with the surface of plate 43. A third plate 46 of springy, electrically conducting material shorter than either the first or second plate is positioned in parallel and electrical contact at one of its ends with the other side of plate 43 at the shorter end thereof; plate 46 is provided with a double bend approximately midway of its length, which bend lies generally parallel with the congruent bend of plate 43, so that the free end of plate 46 is substantially parallel with the longer end of plate 43, but not in contact therewith and extending approximately only to the middle region of the brush-carrier assembly. A brush 47 of suitable electrically conducting material is mounted on the free end of plate 46; a second brush 48 of electrically conducting material is mounted on the longer end of plate 43. The dimensions of the brushes are selected so that they extend outward ly from their respective mountings to the same plane; in practice this plane is achieved by lapping the brush faces in the assembled brush-carrier simultaneously against a flat plate. The distance between the brushes is such that when a carrier is properly angularly mounted on the wiper arm one brush will be in electrical contact with one of the contact rings, and the other brush will be in electrical contact with a circular series of interrupted contacts; as previously mentioned, the brushes may be so oriented at the mounting operation that their longest dimensions are tangent to the travel circle (shown in Fig. 6). A damping element 49 of appropriate resilient material, such as rubber for example, is positioned between the adjacent but non-contacting portions of plates 43 and 44 securely connected to each of said plates, as by adhesive bonding or other suitable means.

It will be seen from the foregoing description that the combination of a plurality of spring arms connected by a resilient damping element prohibits vibration of the brush in harmony with its rate of travel across interrupted contacts. In the instant embodiment it has been deemed unnecessary to damp the shorter arm 46, since it travels on a continuous circular contact where no bouncing will occur, although if desired to use it against interrupted contacts it may be damped in a similar manner.

The selection of suitable conducting materials for contacts, slip-rings, and brushes is important where maximum life is desired, or where free particles of metal dislodged by wear might form undesired conducting paths, or where furring of contacts or brushes would disturb the delicate electrical relationships. Although for some uses any conducting material may be employed, for the high performance of the present invention, it has been found satisfactory to make the interrupted contacts and the slip-rings of silver, and preferably of coin silver, as having the desired electrical properties and being more durable than pure silver. Brushes are preferably made either of an electrical alloy comprising gold, silver, and platinum, or of another electrical alloy comprising silver, copper, cadmium, and nickel.

A further improvement for reducing wear is to plate the contacts and slip-rings with a metal having increased hardness or non-seizing qualities in cooperation with the metals of the brushes, while still maintaining satisfactory electrical properties. It has been found advantageous to plate the rings and contacts with rhodium when it is desired to run brushes of the gold-silver-platinum alloy against them; when running brushes of the silver-coppercadmium-nickel alloy, 2. hard gold plating is beneficial, comprising principally gold but having hardening alloying components such as cadmium, nickel, zinc, or copper, or any combination of such hardening components.

Where a duty cycle of high precision is required, and the on-time of apposition between a brush and a spaced contact must be kept within narrow limits, the dimensions of contacts, spacing therebetween, and length of brushes are carefully pre-selected in terms of the rotational speed which will be employed. It is important, however, that the brushes be longer than the space between contacts, in order that the brushes shall not drop between the contacts, and for such precision uses it is preferable that the length of the brushes be at least slightly more than twice the distance between contacts. Since it may be presumed that the spring-loading urging the brush toward the contacts is exerted in the center of the brush-length, the leading edge might tend to drop and dig into the next contact if a short brush were used. However, if a brush of suflicient length is employed, the point of loading will trail sufficiently to be still over one contact when the leading edge enters the next contact, thus maintaining the brush face in the plane of travel. Further insurance against such gouging may be obtained by providing a slight radius on the leading corner; if this is done, care should be taken that the remaining flat portion on the brush face is sufficiently long to prevent that portion of the brush from dropping between contacts. On the other hand, the brush must not be long enough to touch more than two contacts at one time.

As an example of the foregoing considerations, it has been found convenient in one embodiment of the present invention to make the smaller contacts of the inner circle, for example, .062" in diameter, with a spacing of .015 therebetween. The brush-length employed is .045; assuming that spring-loading is exerted in the center of the brush, the forward end will then be .0225" in length, or more than the spacing of .015". If a radius of .020",

for example, is then placed on the leading edge, the trail-- ing fiat length will still be .025", or more than enough to prevent the brush from dropping between contacts. Naturally, the foregoing dimensions are only such as have been found suitable in one embodiment of the invention, and are not to be understood as in any way limiting it.

Turningnow to Fig. 7, there is shown another embodiment of a brush-carrier, wherein plate 44 and damping element 49 are omitted. Damping in this instance is accomplished by employing in place of plate 44 a similar plate 5 of considerably shorter length, so that its free end which is bent toward plate 43 contacts the latter plate in the center region of its longer section. The two springy plates 43 and 51 have different vibratory periods by reason of their difference in length, the two vibratory periods damping and cancelling each other without loss of resilience by the contact of the shorter plate with the longer one, approximately in the center of the vibrating section thereof.

Damping may be further achieved by the means shown in the embodiment of Fig. 8, wherein a springy plate 52 takes the place of either plate 44 or 51. Plate 52 is of the same length and general configuration as plate 44, but has a different vibratory period from plate 43 because of its greater thickness; it may also be made thinner than plate 43. It is in contact with plate 43 at the free end thereof, and again the two periods damp and cancel each other without loss of resilience.

Referring now to Fig. 9, there is shown a bottom view of any of the brush-carriers, having the brushes not oriented tangent to the travel circle but longitudinally parallel with the spring plates. This embodiment is characterized by greater ease of manufacture than in the previous cases where the brushes were arranged tangentially.

Fig. 10 shows still another embodiment of any of the brush-carriers, wherein brushes 53 are again tangentially disposed, but have a cross-section of substantially teardrop shape of appropriate dimensions, positioned in such a manner that the blunt nose is the leading edge in making contact, and the pointed end comprises the trailing edge. This configuration may be used in applications where it is undesirable to have any corners or other sharp portions at the leading edge.

A preferred embodiment of the brush-carrier is shown in Fig. 11. Plates 43 and 46 are the same as previously described, as are brushes 47 and 48. However, the plates 44, 51, or 52 of previous embodiments are here replaced by springy plate 54, which has no bend, its free end extending approximately as far as the free end of plate 43 and generally parallel therewith. A damping element 49 of resilient material is positioned between the parallel free ends of plates 43 and 54 and secured to each. This embodiment is particularly characterized by greater ease of manufacture than the other brush-carriers, and has proved equally satisfactory in service.

In operation it may be convenient to make electrical connection with each of the pins in a contact circle, taking a continuously changing reading on a recording instrument as circuits are successively completed between the pins and the corresponding slip ring, as the brushes are swept over the contacts. On the other hand, it may be desirable to make connection with alternate pins, leaving every other pin dead, or to gang selected groups of pins with or without dead pins between. There is no limitation on the selection of contacts to be utilized except such. as may be inherent in the size of the device and the number of circuits it is desired to operate. The novel apparatus of the present invention has been successfully operated at speeds as high as 1200 revolutions per minute, with the improved wiper arm providing vibrational damping of such quality that absolutely no hash and no interruption or distortion of signal was observed on an oscilloscope to which the circuits were connected.

Although a few embodiments of the present invention have. been described. in detail, it is understood that various other modifications may be made without departing from the invention, and it is not intended to limit its scope to the arrangements shown and discussed, but only by the spirit of the appended claims.

What is claimed is:

1. In a commutator, rotary switching means comprising in combination a wiper arm having an aperture transversely therethrough midway thereof and attachable thereby to a rotary shaft, and at least one brush-carrier attached to said arm, said brush-carrier comprising a first springy arm bearing a first brush at one end thereof, a second springy arm having a difiierent vibratory period from said first arm and in contact with said first arm at each end but spaced apart therefrom at the middle, and a third springy arm spaced apart from said first arm and bearing a second brush at. one end thereof, said first and third arms being in electrical. contact at the ends opposite said brushes.

2. In a commutator, rotary switching means comprising in combination a wiper arm having an aperture transversely therethrough midway thereof and attachable thereby to a rotary shaft, and a plurality of brush-carriers as in claim 1.

3. In a-commutator, rotary switching means comprising in combination a wiper arm having an aperture transverse ly therethrough adapted to receive a rotatable shaft, means for securing said arm to said shaft, at least one brushcarrier attached to said arm, said brush-carrier bearing a plurality of brushes having contact faces in substantially the same plane, said brushes being oriented longitudinally tangent to the travel circleand having a substantially tear-- drop shape wherein the blunt nose is the leading edge.

4. In a commutator, rotary switching means comprising in combination a wiper arm having an aperture therethrough adapted to receive a rotatable shaft, means for at taching said arm to said shaft, a plurality of brush-carriers having vibration damping means associated therewith, each of said brush-carriers bearing a plurality of brushes and comprising an electrically conducting path between said brushes, and insulating means borne by said arm and adapted to mount said brush-carriers thereon at 'difierent angles to the longitudinal axis thereof, whereby said brushes are positioned at different radial distances from said shaft aperture.

5. In a commutator, rotary switching means as in claim 4, wherein the brush-carriers are angularly adjustable.

6. In a commutator, rotary switching means as in claim 4, wherein the spacing between brushes is the same on each brush-carrier.

7. A commutator comprising in combination a barrel; motor means having a rotatable shaft extending into said barrel coaxial therewith; a wiper arm mounted on said shaft and rotatable therewith in a plane normal to the axis thereof; a switch-plate disposed within said barrel coaxial therewith adjacent said wiper arm and bearing a first circular electrical contact member, a second circular electrical contact member surrounding said first member, a first circular series of spaced-apart contacts surrounding said second member and a second circular series of spacedapart contacts surrounding said first series, all said contact members and said spaced-apart series being mutually concentric and substantially coplanar at their inner surfaces; at first electrically conducting brush-carrier insulatingly mounted toward one end of said wiper arm and having a plurality of springy arms and having rubber vibratio damping means positioned between two of said spri gy arms; a first brush mounted on said first carrier and in elec trical contact with said first circular contact member; a sec ond brush. mounted on said first carrier and adapted to make serial electrical contact with said spaced-apart contacts of said first series; asecond electrically conducting brush-carrier insulatirigly mounted toward the opposite end of said wiper arm and having a plurality of springy arms and having vibration damping means positioned between two of said springy arms; a third brush mounted on said second carrier and in electrical contact with said second circular contact member; and a fourth brush mounted on said second carrier and a daptedto make serial electrical contact with said spaced-apart contacts of said second series; all said brushes being oriented longitudinally tangent to the travel. circle, whereby when said wiper arm is rotated said brushes are traversed across said contacts in a continuous. wiping motion thereby successively completing intermittent circuits between said first circular contact member and said first series, and between said second circular contact and said second series.

8. A commutator comprising in combination a barrel; motor means having a rotatable shaft extending into said barrel coaxial therewith; a wiper arm disposed within said barrel and mounted on said shaft and rotatable therewith in a plane normal to the axis thereof; a switch-plate formed of glass-bonded mica disposed within said barrel coaxial therewith adjacent to said wiper arm and having molded thereinto a first circular gold-plated electrical contact member, a second circular gold-plated electrical contact member surrounding said first member, a first circular series of gold-plated spaced-apart contacts surrounding said second member, and a second circular series of gold-plated spaced-apart contacts surrounding said first series, all said circular members and series being retained mutually concentric and with non-varying spacing and substantially coplanar at their brush-contacting surfaces; a first electrically conducting brush-carrier insulatingly mounted toward one end of said wiper arm and having a plurality of springy arms and having rubber vibrationdamping means positioned between two of said springy arms; a first brush of silver-copper-cadmium-nickel alloy mounted on said first carrier and in sliding contact with said first circular contact member; a second brush of silvercopper-cadmium-niclcel alloy mounted on said first carrier and adapted to make serial electrical contact with said spaced contacts of said first series; a second electrically conducting brush-carrier insulatingly mounted toward the opposite end of said wiper arm and having a plurality of springy arms and having rubber vibration-damping means positioned between two of said springy arms; a third brush of silver-copper-cadmium-nickel alloy mounted on said second carrier and in sliding contact with said second circular contact member; and a fourth brush of silver-copper cadmium-nickel alloy mounted on said second carrier and adapted to make serial electrical contact with said spaced contacts of said second series; all said brushes being oriented longitudinally tangent to the travel circle, whereby when said wiper arm is rotated said brushes are traversed across said contacts in a continuous wiping motion there by successively completing intermittent circuits between said first circular contact and said first series, and between said second circular contact and said second series.

9. A commutator comprising in combination a switchplate molded of glass-bonded mica, said switch-plate having molded thereinto a plurality of circular electrical contacts and a plurality of circular series of electrical contact members, said members of said series being spaced apart a distance of less than half the diameter of individual members, said circular contacts and said spaced contacts being permanently retained by said switchplate in non-varying spacing and with their brush-engaging surfaces in the same plane; a wiper arm having an aperture transversely therethrough; a rotatable shaft extending through said aperture; means for attaching said arm to said shaft; a first resilient brush-carrier insulatingly mounted toward one end of said arm and having rubber vibration damping means associated therewith, said first carrier bearing a first pair of spaced-apart brushes of which one is in continuous contact with one of said circular contacts and the other brush is adapted to make serial contact with the spaced-apart members of one of said series, whereby intermittent circuits are completed through said first carrier; a second resilient brush-carrier insulatingly mounted toward the opposite end of said arm and having rubber vibration-damping means associated therewith, said second carrier bearing a second pair of brushes spaced apart the same distance as said first pair, one brush of said second pair being in continuous contact with another of said circular contacts and the other brush of said second pair being adapted to make serial contact with the spaced-apart members of another of said series, whereby intermittent circuits are completed through said second carrier; each of said brushes adapted to make serial contact being oriented longitudinally tangent to the travel circle and having a length in the arcuate direction at least double the spacing between spaced members; and motor means operatively connected to said rotatable shaft whereby said brushes are circularly traversed across said contacts.

10. A commutator comprising in combination a barrel; motor means having a rotatable shaft extending into said barrel coaxial therewith; a wiper arm disposed within said barrel and mounted on said shaft and rotatable therewith in a plane normal to the axis thereof; a switch-plate formed of glass-bonded mica disposed within said barrel coaxial therewith adjacent to said wiper arm and having molded thereinto a first circular electrical contact, a second circular electrical contact surrounding said first contact, a first circular series of contact members surrounding said second circular contact, and a second circular series of contact members surrounding said first series, said members of said series being spaced apart a distance of less than half the diameter of individual members, all said circular members and series being rhodium-plated and retained mutually concentric and with non-varying spacing and substantially coplanar at their brush-contacting faces; a first electrically conducting brush-carrier insulatingly mounted toward one end of said wiper arm and having a plurality of springy arms and having rubber vibration-damping means positioned between two of said springy arms; a first brush mounted on said first carrier and in sliding contact with said first circular contact; a second brush mounted on said first carrier spaced apart from said first brush and adapted to make serial electrical contact with said spaced members of said first series, said second brush having a length in the arcuate direction at least double the spacing between said spaced members; a second electrically conducting brush-carrier insulatingly mounted toward the opposite end of said wiper arm and having a plurality of springy arms and having rubber vibration damping means positioned between two of said springy arms; a third brush mounted on said second carrier and in sliding contact with said second circular contact; a fourth brush mounted on said second carrier and spaced apart from said third brush the same distance as between said first and second brushes and adapted to make serial electrical contact with said spaced members of said second series, said fourth brush having a length in the arcuate direction at least double the spacing between said spaced contact members; all said brushes being of gold-silver-platinum alloy and oriented longitudinally tangent to the travel circle, whereby when said wiper arm is rotated said brushes are traversed across said circular contacts and said spaced members in a continuous wiping motion thereby successively completing intermittent circuits between said first circular contact and said first series, and between said second circular contact and said second series.

11. A commutator comprising in combination a switch-plate formed of glass-bonded mica, having molded thereinto and protruding from the brush-contacting face thereof at least one circular electrical contact member, electrical conducting means in contact with said circular member and molded into said switch-plate and protruding from the opposite face thereof at least one circular series of spaced-apart electrical pin contacts molded into said switch-plate concentrically with said circular member and having their brush-contacting surfaces in the same plane as the surface of said circular member and having their opposite ends protruding from the opposite face of said switch-plate, adjacent members of said pin contacts being spaced apart by a distance of less than half their diameter, all said pins and said circular member being permanently retained in invariable spacing and in said plane by said switch-plate, rotary switching means having at least one first brush in continuous contact with said circular member and at least one second brush adapted to circular travel across said spaced contacts, said brushes having a length in the direction of travel at least double the amount of spacing between adjacent pin contacts, a conducting path between said first and second brushes whereby circuits are serially established between said circular member and said contacts, and motor means adapted to impart rotary motion to said switching means, the first contacting edge of said brushes being rounded and the remaining flat length of said brushes being at least longer than the spacing between said spaced contacts.

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